I. Field of the Invention
The present invention relates to communication systems, particularly to a plurality of methods for reducing the average transmit power from a sectorized base station.
II. Description of the Related Art
In a code division multiple access (CDMA) cellular telephone system, wireless local loop (WLL), satellite communication system such as GLOBALSTAR, or personal communications system (PCS), a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a mobile unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to. modulate signals transmitted from the base stations and the mobile units. Transmitter stations using different PN codes, or PN codes that are offset in time, produce signals that can be separately received at the receiving station.
In an exemplary CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations. During system operation, the mobile unit is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is equipped with a searching element that allows the mobile unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
A method and system for providing communication with the mobile unit through more than one base station during the handoff process are disclosed in U.S. Pat. No. 5,267,261, issued Nov. 30, 1993, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention which is incorporated herein by this reference. Using this system, communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This type of handoff may be considered as a "soft" handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the mobile unit is in communication with two base stations, a single signal for the end user is created from the signals from each base station by a cellular, WLL, GLOBALSTAR, or PCS controller.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, a mobile unit communicates through a first base station and the Active Set contains only the first base station. The mobile unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile unit communicates a message to the first base station identifying the new base station. A system controller decides whether to establish communication between the new base station and the mobile unit. Should the system controller decide to do so, the system controller sends a message to the new base station with identifying information about the mobile unit and a command to establish communications therewith. A message is also transmitted to the mobile unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The mobile unit searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The system controller receives this message through at least one of the base stations with which the mobile unit is communicating. The controller may decide to terminate communications through the base station having a weak pilot signal strength.
The system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communications are to be terminated. The base stations through which communication is established send the message to the mobile unit. The system controller also communicates information to the base station to terminate communications with the mobile unit. The mobile unit communications are thus routed only through base stations identified in the new Active Set.
Because the mobile unit is communicating with the end user though at least one base station at all times throughout the soft handoff processes, no interruption in communications occurs between the mobile unit and the end user. A soft handoff provides significant benefits in its inherent "make before break" communication over conventional "break before make" techniques employed in other cellular communication systems.
A typical cellular, WLL, GLOBALSTAR, or PCS system contains some base stations having multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas. The process of simultaneous communication with two sectors of the same base station is called softer handoff. The process of soft handoff and the process of softer handoff are the same from the mobile unit's perspective. However the base station operation in softer handoff is different from soft handoff. When a mobile unit is communicating with two sectors of the same base station, the demodulated data signals of both sectors are available for combination within the base station before the signals are passed to the system controller. Because the two sectors of a common base station share circuitry and controlling functions, a variety of information is readily available to sectors of a common base station that is not available between independent base stations. Also two sectors of a common base station send the same power control information to a mobile unit (as discussed below). In satellite communication system such as GLOBALSTAR, most users will be in continuous softer handoff.
In a cellular, WLL, GLOBALSTAR, or PCS system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmitter power of each mobile unit is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each mobile unit may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a mobile unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other mobile units. On the other hand, if the mobile unit transmitted signal is at a power level that is too high when received at the base station, communication with this particular mobile unit is acceptable but this high power signal acts as interference to other mobile units. This interference may adversely affect communications with other mobile units.
Path loss in the radio channel is defined as any degradation or loss suffered by a signal as it travels over-the-air and can be characterized by two separate phenomena: average path loss and fading. The forward link, i.e., the link from the base station to the mobile unit, typically but not necessarily operates on a different frequency than the reverse link, i.e., the link from the mobile unit to the base station. Nevertheless, because the forward and reverse link frequencies are within the same frequency band, a significant correlation exists between the average path loss of the two links. For example, a typically cellular system has one of its forward link channels centered about 882 MHz paired with one of its reverse link channels centered about 837 MHz. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time. The characteristics of fading on the channel are the same, however, for both the forward and reverse link because the frequencies are within the same frequency band. Therefore, the average of channel fading over time for both links is typically the same.
In an exemplary CDMA system, each mobile unit estimates the path loss of the forward link based on the total power at the input to the mobile unit. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the mobile unit. From the estimate of the average forward link path loss, the mobile unit sets the transmit level of the reverse link signal. Should the reverse link channel for one mobile unit suddenly improve compared to the forward link channel for the same mobile unit due to independent fading of the two channels, the signal as received at the base station from this mobile unit would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus a rapid response of the mobile unit transmit power to the sudden improvement in the channel would improve system performance.
Mobile unit transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is in communication measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile unit. A power adjustment command is generated by each base station and sent to the mobile unit on the forward link. In response to the base station power adjustment command, the mobile unit increases or decreases the mobile unit transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved.
When a mobile unit is in communication with more than one base station, power adjustment commands are provided from each base station. The mobile unit acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other mobile unit communications and yet provide sufficient power to support communication from the mobile unit to at least one of the base stations. This power control mechanism is accomplished by having the mobile unit increase its transmit signal level only if every base station with which the mobile unit is in communication requests an increase in power level. The mobile unit decreases its transmit signal level if any base station with which the mobile unit is in communication requests that the power be decreased. A system for base station and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued Oct. 8,1991, assigned to the Assignee of the present invention.
Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit communicates with each base station through which communication is possible, typically between one to three base stations although a greater number is possible. In doing so, the mobile unit avoids inadvertently interfering with communications through a base station receiving the mobile unit's signal at an excessive level but unable to communicate a power adjustment command to the mobile unit because communication is not established therewith.
As a mobile unit moves toward the edge of the base station coverage area, the signal strength of the forward link signal at the mobile unit falls. Also as the mobile unit moves to the edge of the coverage area of the current base station, the mobile unit generally moves closer to the base station coverage area of other base stations. Thus as the mobile unit moves toward the edge of the base station coverage area, the signal level from the current base station falls while interference from other base stations increases. The falling signal level also means that the signals are more sensitive to thermal noise and the noise created by the receiving circuitry in the mobile unit. This situation may be aggravated by a mobile unit located within an area where two sectors of a common base station overlap.
In a case where a set of base stations is operating at near capacity, the mobile unit located at the edge of the base station coverage area and within the coverage area of two sectors of the same base station may experience a drop in signal-to-noise ratio such that the quality of communications is degraded. The softer handoff process provides information to the base station that can be used to improve this situation. The improvement can be created by decreasing the average power transmitted by each sector of a base station. By decreasing the average power transmitted by each sector of a base station, the interference to all mobile units is reduced. Thus the interference to the mobile units at the edge of the coverage area is also reduced causing an increase in the average signal-to-noise ratio of mobile units a the edge of the coverage area.
In a system having soft and softer handoff capabilities and having neighboring base stations at or near capacity, a given amount of base station power is divided among the forward link signals such that each additional forward link signal transmitted from a base station decreases the power of other forward link signals. In a system operating at capacity, compare a base station having two sectors in which every mobile unit in the coverage area of the base station is in softer handoff mode to a base station having two sectors in which no mobile unit is in softer handoff mode. In the base station having every mobile unit in softer handoff, each forward link signals from each sector is transmitted at one-half of the power of each forward link signal from the base station having no mobile unit in softer handoff. Because, in the case where every mobile unit is in softer handoff, the signals from each sector are combined in the mobile unit, the signal-to-interference ratio after combining is equal to the no handoff case if and only if each mobile unit is well served by both sectors. However, in reality not every mobile unit in softer handoff in a sectorized base station is well served by each sector.
The present invention is a technique which could be used to reduced the number of signal transmitted by a sector. A fewer number of signals to be transmitted from a sector means more power available for the remaining signals. When the base station transmits higher power forward link signals, the signal-to-interference for mobile units operating at handoff boundaries or at the edge of the coverage area is improved. Alternatively, as the number of signals is reduced, the total base station transmitter power can decrease which also results in less interference power in the system. These techniques can be used to reduce the number of mobile units in soft and softer handoff.
It is therefore the object of the present invention to improve forward link signal-to-interference ratio by reducing ineffectual forward link transmissions to mobile units in soft or softer handoff, reducing the interference power to other mobiles and making more transmitter power available to all useful links to mobile units.
It is therefore the object of the present invention to provide a plurality of methods to decrease the transmission power from a base station.
It is another object of the present invention to provide a plurality of methods to provide an enhanced softer handoff process to improve the forward link performance.